Hydroperoxide lyase (HPL) cleaves lipid hydroperoxides to produce volatile flavor molecules and also potential signal molecules. We have characterized a gene from Arabidopsis that is homologous to a recently cloned HPL from green pepper (Capsicum annuum). The deduced protein sequence indicates that this gene encodes a cytochrome P-450 with a structure similar to that of allene oxide synthase. The gene was cloned into an expression vector and expressed in Escherichia coli to demonstrate HPL activity. Significant HPL activity was evident when 13S-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid was used as the substrate, whereas activity with 13S-hydroperoxy-9(Z),11(E)-octadecadienoic acid was approximately 10-fold lower. Analysis of headspace volatiles by gas chromatography-mass spectrometry, after addition of the substrate to E. coli extracts expressing the protein, confirmed enzyme-activity data, since cis-3-hexenal was produced by the enzymatic activity of the encoded protein, whereas hexanal production was limited. Molecular characterization of this gene indicates that it is expressed at high levels in floral tissue and is wound inducible but, unlike allene oxide synthase, it is not induced by treatment with methyl jasmonate.In green plant tissue HPL cleaves C18-lipid hydroperoxides to form a C6-aldehyde and a 12-carbon oxoacid (Hatanaka, 1993). The C12 product of HPL leads to the formation of traumatin, which is implicated in wound signaling (Zimmerman and Coudron, 1979). The C6-aldehyde products of the HPL reaction depend on the substrate; cis-3-hexenal is formed from HPOT and hexanal is formed from HPOD. The lipid hydroperoxide substrates were generated from C18 free fatty acids by the enzymatic activity of lipoxygenase (see Fig. 1). HPL activity has been found in a variety of plants and is thought to be associated with the chloroplast envelope (Blée and Joyard, 1996), developmentally regulated (Vick and Zimmerman, 1976;Gardner et al., 1991;Riley et al., 1996), and discernible as two distinct activities in tea leaves (Matsui et al., 1991).Although the central lipoxygenase pathway is present in animal systems, and the formation of prostaglandins and leukotrienes in animals is analogous to the formation of jasmonates in plant tissue (Anderson, 1989), there is no pathway in animals analogous to the HPL branch pathway. The C6 compounds produced by HPL are released rapidly from disrupted plant tissue and form the basis for the "green note" flavor characteristic of plant tissue. The green note flavor is an important determinant of fresh fruit and vegetable quality, and C6 volatiles are widely used as a prepared food additive (Hatanaka, 1993). C6 volatiles produced from this pathway also have antimicrobial properties, suggesting that they may play a protective role in plant defense (Croft et al., 1993). They may also have an application as antimicrobial fumigants in postharvest storage (Archbold et al., 1997). In addition, C6 volatiles of the HPL pathway induce phytoalexin accumulation (Zeringue, 1992) and inhi...
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